An ex-girlfriend testified yesterday that skinhead Thomas Gibison discussed "numerous times" that he had killed a black man in Philadelphia to earn a spider-web tattoo.

Gibison "said he would never forget the slapping sound of the guy's head hitting the ground," testified the 39-year-old Newark, Del., woman. After the killing, she said, Gibison and a pal "melted the weapon down."

In tears, the ex-girlfriend testified she feared Gibison would kill her if she revealed the slaying.

Not until she suffered Gibison's hours-long torture six years after the 1989 killing while two friends "watched and wouldn't help" did she realize her onetime beau cared for no one.

The woman testified that Gibison nearly strangled her and raped her with objects while she was handcuffed for hours. He also made her watch while he shot and bashed her pit bull to death with a baseball bat. Because of the alleged assault, the Daily News is withholding her name.

Yesterday was the second day of an expected weeklong murder trial for Gibison, 38, of Newark, Del., before Common Pleas Judge M. Teresa Sarmina.

In the last trial of his 35-year career as an assistant district attorney, Roger King is prosecuting Gibison on charges of murder, criminal conspiracy, ethnic intimidation and weapon offenses in the April 16, 1989, killing of Aaron Wood in North Philadelphia.

The ex-girlfriend's dramatic, sobbing testimony before Common Pleas Judge M. Teresa Sarmina opened a window into the sick, sadistic world of the Nazi-loving, race-hating skinhead covered with racist tattoos, which jurors could not see on the defendant, but were able to see them in photos or hear about them in testimony.

When she took the witness stand, her lips quivered at the mention of Gibison's name. As King gently questioned the woman, tears flowed while she recalled the nightmare of Gibison's torture, which was provoked by her dating someone else.

"He beat me in my head, with his fist, anywhere he could reach. I was running around a table, whatever he could reach," he tried to punch, she said.

"He said he was going to kill me," she testified. "He had my neck in his hands, my neck was over the dryer and he was going to snap my neck.

"His grandfather came in and saved my life," she added.

Sobbing, the woman testified about an unspeakable rape with objects later the same night that lasted an estimated six hours.

"He made me strip and handcuffed me. He raped me" in two ways, she said, wiping away tears. "I was screaming and crying for him to stop. He told me if I ever told police, he would kill me."

The next day, on Nov. 19, 1995, her mother took her to Christiana Hospital in Newark, where she was treated and released for cuts and bruises of her swollen face and body.

At the hospital, she testified, she claimed that she had gotten into a fight with another girl and refused to file a police report.

"If they found out my boyfriend beat me, [Gibison] would come after me," she added.

Richard Iardella, a retired detective with the Wilmington Police Department, testified earlier that the woman's ex-husband contacted him about the beating.

When the detective approached the ex-girlfriend, she began to open up because: "My conscience bothered me.

"I told him about the spider-web tattoo. I told him that Tom had committed a murder. At the time, I was more concerned about my rape, than the murder," she testified.

After she recovered, the woman said she spoke frequently to Iardella. She testified she told him that Tom "wanted to get a red teardrop in his spider-web tattoo - one [teardrop] for each person killed." Gibison and co-conspirator Craig Peterson "were talking about how they were going to get them," she added.

At that time, Gibison "belonged to the Ku Klux Klan," she said. But while the couple was together, she added, she did not know of Gibison's affiliations, other than dressing like other skinheads and attended their events.

With the skinheads, he and his pals wore bomber jackets, shaved their heads, wore racist tattoos and T-shirts and listened to skinhead music, she said.

Shown a photo of Gibison, which was obtained by her parents, the ex-girlfriend testified, "It's a picture of Tom, all pumped up, showing me his tattoo on his tummy."

The tattoo shows a skull and a man holding a gun, with the words: "What goes around, comes around." *